Perpendicular Magnetic Recording Media with Magnetic Anisotropy Gradient and Local Exchange Coupling

ABSTRACT

A perpendicular magnetic recording medium adapted for high recording density and high data recording rate comprises a non-magnetic substrate having at least one surface with a layer stack formed thereon, the layer stack including a perpendicular recording layer containing a plurality of columnar-shaped magnetic grains extending perpendicularly to the substrate surface for a length, with a first end distal the surface and a second end proximal the surface, wherein each of the magnetic grains has: (1) a gradient of perpendicular magnetic coercivity H k  extending along its length between the first end and second ends; and (2) predetermined local exchange coupling strengths along the length.

FIELD OF THE INVENTION

The present invention relates to improved perpendicular magneticdata/information storage and retrieval media. The invention hasparticular utility in the design and use of hard disk media comprisinggranular perpendicular-type magnetic recording layers.

BACKGROUND OF THE INVENTION

Magnetic media are widely used in various applications, particularly inthe computer industry for data/information storage and retrievalapplications and in consumer electronics, typically in disk form, andefforts are continually made with the aim of increasing the arealrecording density, i.e., bit density of the magnetic media. Conventionalthin-film type magnetic media, wherein a fine-grained polycrystallinemagnetic alloy layer serves as the active recording layer, are generallyclassified as “longitudinal” or “perpendicular”, depending upon theorientation of the direction of the magnetic anisotropy of the grains ofmagnetic material.

Perpendicular recording media have been found to be superior tolongitudinal media in achieving very high bit densities withoutexperiencing the thermal stability limit associated with the latter. Inperpendicular magnetic recording media, residual magnetization is formedin a direction perpendicular to the surface of the magnetic medium,typically a layer of a magnetic material on a suitable substrate. Veryhigh to ultra-high linear recording densities are obtainable byutilizing a “single-pole” magnetic transducer or “head” with suchperpendicular magnetic media.

Efficient, high bit density recording utilizing a perpendicular magneticmedium requires interposition of a relatively thick (as compared withthe magnetic recording layer), magnetically “soft” underlayer (“SUL”),i.e., a magnetic layer having a relatively lower coercivity of about1-1,000 Oe, such as of a NiFe alloy (Permalloy), between a non-magneticsubstrate, e.g., of glass, aluminum (Al) or an Al-based alloy, and amagnetically “hard” recording layer having relatively high coercivity,typically about 3-50 kOe, e.g., of a cobalt-based alloy (e.g., a Co—Cralloy such as CoCrPtB, CoCrPtTaB, etc.) having perpendicular anisotropy.The magnetically soft underlayer serves to guide magnetic flux emanatingfrom the head through the magnetically hard perpendicular recordinglayer.

A typical conventional perpendicular recording system 20 utilizing avertically oriented magnetic medium 21 with a relatively thick softmagnetic underlayer, a relatively thin hard magnetic recording layer,and a magnetic transducer head 16, is illustrated in FIG. 1, whereinreference numerals 10, 11, 4, 5, and 6, respectively, indicate anon-magnetic substrate, an optional adhesion layer, a soft magneticunderlayer, at least one non-magnetic seed layer (sometimes referred toas an “intermediate” layer or as an “interlayer”), and at least onemagnetically hard perpendicular recording layer with its magnetic easyaxis substantially perpendicular to the film plane.

Still referring to FIG. 1, reference numerals 7 and 8, respectively,indicate the main (writing) and auxiliary poles of the magnetictransducer head 16. The relatively thin interlayer 5, comprised of oneor more layers of non-magnetic materials, serves to (1) prevent magneticinteraction between the soft underlayer 4 and the at least one hardrecording layer 6; and (2) promote desired microstructural and magneticproperties of the at least one magnetically hard recording layer.

As shown by the arrows in the figure indicating the path of the magneticflux φ, flux φ is seen as emanating from the main writing pole 7 ofmagnetic transducer head 16, entering and passing through the at leastone vertically oriented, magnetically hard recording layer 5 in theregion below main pole 7, entering and traveling within soft magneticunderlayer (SUL) 3 for a distance, and then exiting therefrom andpassing through the at least one perpendicular hard magnetic recordinglayer 6 in the region below auxiliary pole 8 of transducer head 16. Thedirection of movement of perpendicular magnetic medium 21 pasttransducer head 16 is indicated in the figure by the arrow above medium21.

With continued reference to FIG. 1, vertical lines 9 indicate grainboundaries of polycrystalline layers 5 and 6 of the layer stackconstituting medium 21. Magnetically hard main recording layer 6 isformed on interlayer 5, and while the grains of each polycrystallinelayer may be of differing widths (as measured in a horizontal direction)represented by a grain size distribution, they are generally in verticalregistry (i.e., vertically “correlated” or aligned).

Completing the layer stack is a protective overcoat layer 14, such as ofa diamond-like carbon (DLC), formed over hard magnetic layer 6, and alubricant topcoat layer 15, such as of a perfluoropolyether (PFPE)material, formed over the protective overcoat layer.

Substrate 10 is typically disk-shaped and comprised of a non-magneticmetal or alloy, e.g., Al or an Al-based alloy, such as Al—Mg having aNi—P plating layer on the deposition surface thereof, or alternativelysubstrate 10 is comprised of a suitable glass, ceramic, glass-ceramic,polymeric material, or a composite or laminate of these materials.Optional adhesion layer 11, if present, may comprise an up to about 200Å thick layer of a material such as Ti, a Ti-based alloy, Cr, or aCr-based alloy. Soft magnetic underlayer 4 is typically comprised of anabout 100 to about 4,000 Å thick layer of a soft magnetic material,which, for example, may be selected from the group consisting of Ni,NiFe (Permalloy), Co, CoZr, CoZrCr, CoZrNb, CoFeZrNb, CoFe, Fe, FePt,FeBNi, FeN, FeSiAl, FeSiAlN, FeCoB, FeCoC, etc. Interlayer 5 typicallycomprises an up to about 300 Å thick layer or layers of non-magneticmaterial(s), such as Ru, TiCr, Ru/CoCr₃₇Pt₆, RuCr/CoCrPt, etc.; and theat least one magnetically hard perpendicular recording layer 6 istypically comprised of about 50 to about 250 Å thick layer(s) of, forexample, Co-based alloy(s) or FePt intermetallic compounds with L1₀structure and including one or more elements selected from the groupconsisting of Cr, Fe, Ta, Ni, Mo, Pt, V, Nb, Ge, B, N, C, and Pd.

A currently employed way of classifying magnetic recording media is onthe basis by which the magnetic grains of the recording layer aremutually separated, i.e., segregated, in order to physically andmagnetically de-couple the grains and provide improved media performancecharacteristics. According to this classification scheme, magnetic mediawith Co-based alloy magnetic recording layers (e.g., CoCr alloys) areclassified into two distinct types: (1) a first type, whereinsegregation of the grains occurs by diffusion of Cr atoms of themagnetic layer to the grain boundaries of the layer to form Cr-richgrain boundaries, which diffusion process requires heating of the mediasubstrate during formation (deposition) of the magnetic layer; and (2) asecond type, wherein segregation of the grains occurs by formation ofoxides, nitrides, and/or carbides at the boundaries between adjacentmagnetic grains to form so-called “granular” media, which oxides,nitrides, and/or carbides may be formed by introducing a minor amount ofat least one reactive gas containing oxygen, nitrogen, and/or carbonatoms (e.g. O₂, N₂, CO₂, etc.) to the inert gas (e.g., Ar) atmosphereduring sputter deposition of the Co alloy-based magnetic layer. Thelatter process does not require heating of the substrate to an elevatedtemperature.

Magnetic recording media with granular magnetic recording layers possessgreat potential for achieving very high and ultra-high areal recordingdensities. An advantage afforded by granular recording layers issignificant suppression of media noise due to great reduction in theexchange coupling between adjacent magnetic grains, resulting from thepresence of non-magnetic material, typically an oxide material, at thegrain boundaries. As indicated above, current methodology formanufacturing granular-type magnetic recording media involves reactivesputtering of a target comprised of the ferromagnetic material for themagnetic recording layer (typically a Co-based alloy) in a reactivegas-containing atmosphere, e.g., an atmosphere comprising oxygen or acompound of oxygen, in order to incorporate oxides in the deposited filmor layer and achieve smaller and more isolated magnetic grains. Granularmagnetic layers formed in this manner have a reduced saturationmagnetization (M_(s)) due to the oxide formation and consumption of acertain amount of the Co component of the ferromagnetic alloy.Alternatively, a target comprised of the ferromagnetic material(typically a Co-based alloy) and the oxide material may be directlysputtered in an inert atmosphere or an atmosphere comprising oxygen or acompound of oxygen. However, the oxide material sputtered from thetarget is subject to decomposition in the environment of the sputteringgas plasma, and, as a consequence, a certain amount of the Co componentof the ferromagnetic alloy is again consumed.

In order to continually increase the bit density of recording over thenext decade, it will be necessary to achieve a corresponding continualdecrease of the dimensions of the magnetic grains in order to maintain agood signal-to-noise ration (SNR) of the magnetic media. Therefore, inpractice, it will be necessary to decrease the grain volume as thedesired linear recording density increases in order to maintain a usableSNR. Such reduction in magnetic grain size, however, will result ingrain sizes which approach the so-called superparamagnetic limit ofmagnetic particles and thereby limit the ability of the media to retainstored information without significant thermal decay. A significantfactor with thermal decay associated with grain sizes approaching thesuperparamagnetic limit is the steepness of onset of the thermal decay.In this regard, it has been estimated that at a certain point a 15%decrease of grain diameter can result in a reduction of storage lifetimeof the media from about 20 years to as little as 1 day.

One proposal for overcoming the superparamagnetic limit is to raise theenergy barrier to thermal reversal of grain magnetization by developmentof media with higher coercivity. However, such approach is problematicin designing high data recording rate media because media with very highcoercivities greater than about 10,000 Oe cannot be accurately writtento by means of the head fields provided by currently availableread/write transducers. This is especially true in high frequencyrecording applications because of a drastic increase in dynamiccoercivity, resulting in inability of the write field to function athigh frequency, leading to incomplete magnetization reversal and causingsignificant increases in noise level and error rate.

Since the early 1990's, advanced magnetic media have been designed andfabricated for achieving better SNR's. For example, dual layerlongitudinal CoNiCr/CoCrTa and dual layer CoCrPt/CoCrPtSi gradient mediawere fabricated in order to enhance the SNR. Such dual layer mediaactually are gradient systems wherein the top (upper) layer provides themedia with high coercivity and the lower layer is optimized for reducingmedia noise.

The ever-increasing need for disk drive media and systems with higherstorage capacities, faster data read/write rates, and lower costs form atriad of conflicting and competing requirements for designing,developing, and fabricating the next generation of disk drives. As aconsequence, the magnetic recording media design practice faces a numberof challenges extending magnetic recording technology to its limits.

Inasmuch as perpendicular magnetic recording media are expected toremain the predominant type of magnetic media for use in disk drives forat least the foreseeable future (e.g., 5-6 years), unique design andengineering schemes are considered necessary for fabrication of improvedperpendicular media capable of meeting future challenges andrequirements for high recording density, high data recording rate diskdrive applications.

In view of the foregoing, there exists a clear need for a new avenue orapproach for the engineering and development of advanced perpendicularmagnetic recording media which achieves the goals of high linearrecording density and high data recording rate without significant lossof thermal stability.

The present invention addresses and solves the need for engineering anddevelopment of improved, high performance advanced perpendicularmagnetic recording media suitable for use in disk drives, comprising anovel combination of gradient magnetic properties and local verticalexchange coupling, while maintaining full compatibility with allrequirements of cost-effective automated fabrication processing.

DISCLOSURE OF THE INVENTION

An advantage of the present invention is improved perpendicular magneticrecording medium adapted for high recording density and high datarecording rate.

Another advantage of the present invention is an improved method forperforming magnetic data/information storage and retrieval at a highrecording density and high data recording rate.

Still another advantage of the present invention is an improved methodfor fabricating magnetic data/information storage and retrieval mediahaving a high recording density and high data recording rate.

Additional advantages and other features of the present invention willbe set forth in the description which follows and in part will becomeapparent to those having ordinary skill in the art upon examination ofthe following or may be learned from the practice of the presentinvention. The advantages of the present invention may be realized andobtained as particularly pointed out in the appended claims.

According to an aspect of the present invention, the foregoing and otheradvantages are obtained in part by an improved perpendicular magneticrecording medium adapted for high recording density and data recordingrate, comprising:

(a) a non-magnetic substrate having at least one surface; and

(b) a layer stack formed on the surface of the substrate, the layerstack including a perpendicular recording layer containing a pluralityof columnar-shaped magnetic grains extending perpendicularly to thesubstrate surface for a length, with a first end distal the substratesurface and a second end proximal the substrate surface, wherein each ofthe magnetic grains:

(1) has a gradient of perpendicular magnetic coercivity H_(k) extendingalong its length between the first and second ends; and

(2) has predetermined local exchange coupling strengths along itslength.

In accordance with embodiments of the present invention, application ofan external magnetic field to the recording layer induces a progressivereversal process of an initial magnetization direction of each of theplurality of columnar-shaped magnetic grains which originates at one ofthe ends, progresses toward the other end, and results in reversal ofthe initial magnetization direction to yield a final magnetizationdirection.

According to preferred embodiments of the invention, the perpendicularmagnetic coercivity decreases from the first end to the second end ofeach of the grains.

Preferably, each of the magnetic grains comprises a plurality ofoverlying sub-layers of magnetic material, each of the sub-layers ofmagnetic material having a different perpendicular magnetic coercivitywhich progressively decreases from a sub-layer at the first end to asub-layer at the second end of each of the magnetic grains.

In accordance with embodiments of the present invention, tailoring ofthe local exchange coupling strength between adjacent sub-layers toachieve a desired coupling strength is accomplished by utilizing one ormore of the following means: (1) a non-magnetic, paramagnetic, orsuperparamagnetic spacer layer of selected thickness positioned at theinterface between the adjacent sub-layers; (2) adjacent sub-layerspositioned in direct contact; and (3) a magnetic layer of selectedthickness positioned between adjacent magnetically hard sub-layers.

According to embodiments of the invention, each of the magnetic grainscomprises two overlying sub-layers with different magnetic materialcomposition, a first sub-layer at the first end of each of the magneticgrains is comprised of CoCrX₁ first magnetic material, where X₁ is atleast one element selected from the group consisting of Ta, Pt, B, V, C,Nd, Cu, Zr, Fe, P, O, Si, and Ni, with a magnetic moment M_(r) fromabout 200 to about 800 emu/cc, a relatively high perpendicularcoercivity H_(k) from about 8,000 to about 20,000 Oe, a thickness δ₁from about 6 to about 25 nm, and a grain size from about 4 to about 10nm; a second sub-layer at the second end of each of the magnetic grainsis comprised of CoX₂ second magnetic material, where X₂ is at least oneelement selected from the group consisting of C, B, Cr, Pt, O, Fe, Ta,Cu, Nd, Ni, and Ti, with a magnetic moment M_(r) from about 400 to about900 emu/cc, a relatively low perpendicular coercivity H_(k) from about1,000 to about 9,000 Oe, a thickness δ₂ from about 3 to about 15 nm, anda crystal structure and grain size matching those of the firstsub-layer; and the total thickness δ₁+δ₂ of the first and secondsub-layers is less than the exchange coupling distances of the magneticmaterials, whereby domain walls are not present in the magnetic grains.According to this embodiment, a non-magnetic spacer layer is present atan interface between the first and second sub-layers for providing aninterfacial coupling strength between the first and second sub-layersfrom about 10⁻² to about 10⁻⁹ erg/cm, the spacer layer having athickness up to about 5 nm and comprised of at least one non-magneticelement selected from the group consisting of Cr, Pt, Cu, Zr, V, C, Ru,Ta, and Si.

Further embodiments of the present invention include those wherein eachof the magnetic grains comprises three overlying sub-layers withdifferent magnetic material composition, wherein the relatively highperpendicular magnetic coercivity H_(k1) of a first sublayer at thefirst end is about 12,000 Oe, the relatively low perpendicular magneticcoercivity. H_(k3) of a third sub-layer at the second end is about 3,000Oe, the perpendicular magnetic coercivity H_(k2) of a second sub-layerintermediate the first and third sub-layers is about 9,000 Oe, and thethickness of each of the three sub-layers is about 6-8 nm.

Still other embodiments of the present invention include those whereineach of the magnetic grains comprises four overlying sub-layers withdifferent magnetic material compositions, the relatively highperpendicular magnetic coercivity H_(k1) of a first sub-layer at thefirst end being about 12,000 Oe, the relatively low perpendicularmagnetic coercivity H_(k4) of a fourth sub-layer at the second end beingabout 3,000 Oe, the perpendicular magnetic coercivity H_(k2) of a secondsub-layer adjacent the first sub-layer being about 9,000 Oe, theperpendicular magnetic coercivity H_(k3) of a third sub-layer adjacentthe second sub-layer being about 6,000 Oe and the thickness of each ofthe four sub-layers is about 5 nm.

In accordance with preferred embodiments of the present invention, theperpendicular recording layer is a granular type layer, and the layerstack comprises a magnetically soft underlayer (SUL) intermediate therecording layer and the substrate surface.

Another aspect of the present invention is an improved method forperforming magnetic data/information storage and retrieval at a highrecording density and high data recording rate, comprising steps of:

(a) providing a magnetic recording medium comprising:

-   -   (i) a non-magnetic substrate having at least one surface; and    -   (ii) a layer stack formed on the surface of the substrate, the        layer stack including a perpendicular recording layer containing        a plurality of columnar-shaped magnetic grains extending        perpendicularly to the substrate surface for a length, with a        first end distal the substrate surface and a second end proximal        the substrate surface, wherein each of the columnar-shaped        magnetic grains has a gradient of perpendicular magnetic        coercivity H_(k) extending along its length between the first        and second ends, and predetermined local exchange coupling        strengths along its length;        and

(b) applying an external magnetic field to the recording layer toreverse an initial magnetization direction of each of the plurality ofcolumnar-shaped magnetic grains to yield a final magnetizationdirection.

According to embodiments of the invention, step (b) comprises inducing amagnetization reversal process of the initial magnetization direction ofeach of the plurality of magnetic grains which originates at one of theends and progresses toward the other of the ends to result in reversalof the initial magnetization direction to yield a final magnetizationdirection.

According to embodiments of the present invention, step (a) preferablycomprises providing a magnetic recording medium wherein theperpendicular magnetic coercivity decreases from the first end to thesecond end of each of the grains.

Preferably, step (a) comprises providing a magnetic recording mediumwherein each of the magnetic grains comprises a plurality of overlyingsub-layers of magnetic material, each of the sub-layers of magneticmaterial having a different perpendicular magnetic coercivity whichprogressively decreases from a sub-layer at the first end to a sub-layerat the second end of each of the magnetic grains.

In accordance with embodiments of the present invention, step (a)comprises providing a magnetic recording medium wherein tailoring of thelocal exchange coupling strength between adjacent sub-layers to achievea desired coupling strength is accomplished by utilizing one or more ofthe following approaches: (1) positioning a non-magnetic, paramagnetic,or superparamagnetic interfacial spacer layer of selected thickness atthe interface between the adjacent sub-layers; (2) forming the adjacentsub-layers in direct contact; and (3) positioning a magnetic layer ofselected thickness between adjacent magnetically hard sub-layers.

Still another aspect of the present invention is a method of fabricatinga magnetic data/information storage and retrieval medium having highrecording density and high data recording rate, comprising steps of:

(a) providing a non-magnetic substrate having at least one surface; and

(b) forming a layer stack on the at least one surface, the layer stackincluding a soft magnetic underlayer (SUL) and an overlyingperpendicular recording layer containing a plurality of columnar-shapedmagnetic grains extending for a length perpendicularly to the substratesurface, with a first end distal the substrate surface and a second endproximal the substrate surface, wherein each of the columnar-shapedmagnetic grains has a gradient of perpendicular magnetic coercivityH_(k) extending along its length between the first and second ends, andpredetermined local exchange coupling strengths along its length.

According to embodiments of the present invention, step (a) comprisesforming a perpendicular recording layer wherein the perpendicularmagnetic coercivity decreases from the first end to the second end ofeach of the grains.

Embodiments of the present invention include those wherein step (a)comprises forming a perpendicular recording layer wherein each of themagnetic grains comprises a plurality of overlying sub-layers ofmagnetic material, each of the sub-layers of magnetic material having adifferent perpendicular magnetic coercivity which progressivelydecreases from a sub-layer at the first end to a sub-layer at the secondend of each of the magnetic grains.

In accordance with certain embodiments of the present invention, step(a) comprises forming a perpendicular recording layer wherein tailoringof the local exchange coupling strength between adjacent sub-layers toachieve a desired coupling strength is accomplished by utilizing one ormore of the following approaches: (1) positioning a non-magnetic,paramagnetic, or superparamagnetic interfacial spacer layer of selectedthickness at the interface between the adjacent sub-layers; (2) formingthe adjacent sub-layers in direct contact; and (3) positioning amagnetic layer of selected thickness between adjacent magnetically hardsub-layers.

Additional advantages and aspects of the present invention will becomereadily apparent to those skilled in the art from the following detaileddescription, wherein embodiments of the present invention are shown anddescribed, simply by way of illustration of the best mode contemplatedfor practicing the present invention. As will be described, the presentinvention is capable of other and different embodiments, and its severaldetails are susceptible of modification in various obvious respects, allwithout departing from the spirit of the present invention. Accordingly,the drawings and description are to be regarded as illustrative innature, and not as limitative.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of the embodiments of the presentinvention can best be understood when read in conjunction with thefollowing drawings, in which the various features are not necessarilydrawn to scale but rather are drawn as to best illustrate the pertinentfeatures and the same reference numerals are employed throughout fordesignating similar features, wherein:

FIG. 1 schematically illustrates, in simplified cross-sectional view, aportion of a magnetic recording, storage, and retrieval system accordingto the conventional art, comprised of a perpendicular magnetic recordingmedium and a single pole transducer head;

FIG. 2 schematically illustrates the magnetism reversal mechanism of themagnetically hard perpendicular magnetic recording layer of theconventional perpendicular medium of FIG. 1;

FIG. 3 schematically shows the quasi-incoherent (“buckling”)magnetization reversal process of a magnetically hard perpendicularrecording layer of a perpendicular medium according to the invention,comprised of a stack of 4 sub-layers;

FIG. 4 schematically illustrates a perpendicular recording layeraccording to embodiments of the invention wherein a non-magnetic spacerlayer positioned at the interface between adjacent sub-layers isutilized for tailoring the local exchange coupling strength between thesub-layers;

FIG. 5 schematically illustrates a perpendicular recording layeraccording to embodiments of the invention wherein layers of softmagnetic material are positioned between adjacent sub-layers ofmagnetically hard material for tailoring the local exchange couplingstrength between the sub-layers; and

FIGS. 6(A)-6(B), respectively, graphically show the magnetizationdistributions at the written transitions in the case of large and smalldeviation angles of the easy axis of magnetic media.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is based upon recognition by the inventors thatperpendicular magnetic recording media fabricated with a main recordinglayer comprised of columnar-shaped magnetic grains with specificallydesigned gradients of magnetic anisotropy, i.e., gradients ofperpendicular magnetic coercivity (H_(k)), and with local exchangecoupling strengths which provide good writability and signal-to-noiseratio (SNR) at ultra-high recording densities (i.e., >˜250 Gbits/in²)and high data recording rates (i.e., >˜2,000 Mbits/sec.) withoutsignificant sacrifice in thermal stability of the media. Further, it hasbeen determined that all significant performance parameters of the mediacan be controllably optimized via appropriate selection of the H_(k)gradient and local exchange coupling strength(s).

Briefly stated, perpendicular media and systems fabricated according tothe principles of the invention are structurally similar to media 21 andsystem 20 shown in FIG. 1, except that the magnetically hardperpendicular magnetic recording layer 6 is replaced with recordinglayer 6′, which, as indicated above, comprises a plurality ofcolumnar-shaped magnetic grains with specifically designed gradients ofmagnetic anisotropy, i.e., gradients of perpendicular magneticcoercivity (H_(k)), and with preselected local exchange couplingstrengths.

The principles of the present invention will now be described withreference to FIGS. 2 and 3, wherein FIG. 2 illustrates the magnetismreversal mechanisms of the magnetically hard perpendicular magneticrecording layer 6 of a conventional perpendicular medium (such as media21) including a uniform magnetic grain of thickness δ and FIG. 3 showsthe progressive, quasi-incoherent magnetization reversal process (suchas a “buckling” process) of a magnetically hard perpendicular recordinglayer 6′ of a perpendicular medium according to the invention. (In eachof these figures, the direction of magnetization within a grain orsub-layer is indicated by the arrow, and optional interlayer 5 of FIG. 1is omitted for clarity).

More specifically, FIG. 2 shows the coherent magnetization reversal(“rotation”) process within a conventional (i.e., uniform)columnar-shaped magnetic grain which is effected by means of anexternally applied magnetic field from a write head spaced at a distanced from the upper end of the magnetic grain (in the following the headfield gradient is assumed to be Karlqvist type), where the initialmagnetization direction prior to application of the external magneticfield is indicated at T=t_(o) and the final magnetization directionafter application of the external magnetic field is indicated at T=t₁;whereas FIG. 3 shows the quasi-incoherent magnetization reversal process(such as a “buckling” process) of a magnetic grain according to theinvention, comprised of 4 moderately exchange coupled, verticallystacked sub-layers layers M₁, M₂, M₃, and M₄ with respective thicknessesδ₁, δ₂, δ₃, and δ₄, and where the perpendicular magnetic coercivityH_(k) progressively decreases from sub-layer M₁ to sub-layer M₄ (SUL 5is omitted from the figure for clarity). As shown, the magnetizationdirection of each of the sub-layers M₁ to M₄ is the same at T=t_(o)(i.e., the initial magnetization direction before application of anexternal magnetic field from the write head), and magnetization reversal(“rotation”) occurs quasi-incoherently in progressive stages illustratedat T=t₁, T=t₂, and T=t₃. Complete reversal of the initial magnetizationdirection is indicated at T=t₄.

In the above, H_(k1)>H_(k2)>H_(k3)>H_(k4), the topmost sub-layer M₁ hasthe highest switching field, and it is assumed that the permeability ofthe underlying SUL is infinite. Assuming that no interlayer (e.g., suchas layer 5 of FIG. 1) is present, according to the incoherentmagnetization reversal process of the invention, the magnetizationdirection of sub-layers M₁, M₂, M₃, and M₄ occurs sequentially, asillustrated. The magnetization reversal process is initiated at thebottom-most sub-layer M₄ of lowest perpendicular magnetic coercivityH_(k4), and proceeds upwardly in sequence from sub-layer M₄ to theoverlying sub-layers M₃ and M₂ of progressively lower perpendicularmagnetic coercivities H_(k3) and H_(k2), and is ultimately controlled bythe topmost sub-layer M₁ of greatest perpendicular magnetic coercivityH_(k1). More particularly, re-orientation or reversal of themagnetization direction of the entire grain occurs when themagnetization direction of the topmost sub-layer M₁ is completelyreversed (as at T=t₄ in the illustrated case).

Stated differently, when the magnetic grains are comprised of sub-layerswith a coercivity gradient, application of the external writing fieldfrom the head causes the sub-layer with the smallest perpendicularmagnetic coercivity H_(k), i.e., the lowermost sub-layer of the stack,to switch or reverse its magnetization direction. This substantiallysimultaneously induces a quasi-incoherent rotation process in theoverlying sub-layers of higher magnetic coercivity. The magnetizationreversal process in each grain is essentially an incoherent rotationprocess, i.e., a type of induced “quasi-buckling” or “curling” process,which is generated in the lowermost sub-layer with high magnetic momentand relatively lower intrinsic coercivity, via tailored exchangeinteractions. By contrast, coherent magnetization reversal inconventional magnetic grains requires a larger switching field and poormedia writability, resulting in difficulty in obtaining high densityrecording with good writability and thermal stability.

It is also noted that, with the materials conventionally utilized forfabricating high performance magnetic recording media, the intrinsicexchange coupling within the media is usually too strong to allow forany incoherent magnetization reversal as required by the invention.Therefore, according to the inventive methodology, multi-step incoherentmagnetization reversal within the grains is facilitated by suitablytailoring the perpendicular magnetic coercivity of the varioussub-layers to obtain a desired gradient of H_(k) and the local exchangecoupling strengths between adjacent sub-layers. In this regard, it isnoted that the strength of the exchange coupling between adjacentsub-layers and the thickness of each sub-layer play important roles indictating the overall magnetization reversal/re-orientation process. Forexample, if the exchange coupling strength is too small, the overallmagnetization reversal/re-orientation process can become a quasi-fanningprocess which does not afford good thermal stability. On the other hand,if the thickness of the lower-most sub-layer is too little, triggeringof the magnetization reversal process would not be strong enough toinduce incoherent magnetization reversal in the overlying layers if theexchange coupling strength is too high. Tailoring of the local exchangecoupling strength between adjacent sub-layers is therefore necessary inorder to achieve maximum local magnetization reversal torque, and forsignificantly reducing the overall switching field of each grain.

According to the invention, tailoring of the local exchange couplingstrength between adjacent sub-layers to achieve a desired couplingstrength is accomplished by utilizing one or more of the followingapproaches: (1) positioning a non-magnetic, paramagnetic, orsuperparamagnetic spacer layer SL of selected thickness at the interfacebetween adjacent sub-layers, as schematically shown in FIG. 4; (2)forming the adjacent sub-layers in direct contact; and (3) positioning amagnetic layer of selected thickness between adjacent magnetically hardsub-layers, as schematically shown in FIG. 5 for a grain structurecomprised of n stacked sub-layers of magnetically hard material withintervening magnetic layers.

It should be noted that despite apparent differences of approaches(1)-(3), the underlying physics is equivalent, because the fundamentalmagnetic properties of the magnetic material are associated with thedimensionality of the material per se. For quasi one-dimensional andtwo-dimensional thin-film magnetic materials, the weak spin effects willlead to reductions in the anisotropy, magnetic moment, and localexchange coupling strength. Approach (3) has an advantage in thatcontinuity of the microstructure of the magnetic grains is more easilymaintained. Finally, manipulation of the sub-layer thicknesses allowsobtaining of desirable local magnetic properties for achieving optimalrecording performance.

Tailoring of the magnetic anisotropies, i.e., the perpendicular magneticcoercivities H_(k), of each of the sub-layers is accomplished, in knownfashion, as by appropriate selection of the magnetic alloys and theirprocessing conditions; and each of the sub-layers and spacer layers aresequentially epitaxially deposited (by conventional methodologies,including sputtering techniques) so as to replicate the crystalstructure and cross-sectional dimensions of the underlying grains (i.e.,grain sizes) and form a magnetically hard perpendicular recording layercomprised of columnar-shaped magnetic grains extending perpendicularlyto a substrate for a desired length. Granular perpendicular magneticrecording layers embodying the principles of the present invention maybe formed by means of reactive sputtering techniques, as known in theart and described above.

Advantageously, when the magnetization reversal process is incoherentaccording to the invention, the read/write head spacing is reduced, ascompared with the head-media spacing (HMS) with conventional coherentmagnetization reversal. More specifically, in the incoherent case (FIG.3), the HMS is given by (d+δ₁/2), which is much smaller than the dspacing in the coherent case (FIG. 2), which is given by (d+δ₂). Forinstance, if d=6 nm and δ=20 nm in the conventional, coherent reversalcase, and δ₁=5 nm in the incoherent reversal case, the effective HMSwould be 8.5 nm in the incoherent case and 16 nm in the coherent case.As a consequence, the SNR's of the inventive coercivity gradient grainsand conventional, uniform grains will be dramatically different. Forexample, it is conservatively estimated that use of a 3 sub-layerperpendicular magnetic recording layer with coercivity gradientaccording to the invention would provide at least a 1-3 db increase inSNR (facilitating a corresponding increase in recording density) byvirtue of the dramatic decrease in HMS afforded by the invention.

It should be noted that the head field magnetic gradient should be lessthan the gradient of magnetic coercivity of the various sub-layersconstituting the magnetic grains, which requirement places severalconstraints on media design practice, resulting in significant reductionof the effective head-media spacing (HMS), and thus providing a verysubstantial improvement in recording performance. In addition, it shouldbe recognized that coercivity gradient perpendicular media fabricatedaccording to the invention can also advantageously exhibit substantiallyreduced easy axis distributions by virtue of the presence of severalsub-layers within a single columnar-shaped magnetic grain, leading to areduction of the media switching distribution and an increase in themedia nucleation field. In this regard, the number of sub-layers withina grain is not limited to the illustrative embodiments described belowwhich comprise 2, 3, or 4 sub-layers. Rather, the greater the number ofsub-layers within a grain, the smaller the deviation angle of the easyaxis. As a consequence, the resultant magnetization becomes sharperand/or more symmetric at the written transition locations. For example,FIGS. 6(A)-6(B), respectively, graphically show the magnetizationdistributions at the written transitions in the case of large and smalldeviation angles of the easy axis, wherein it is evident that theresultant magnetization becomes sharper and/or more symmetric at thewritten transition locations when the deviation angles of the easy axisare smaller.

Additional advantages of the inventive media include reduced grain sizedistributions and the ability to fabricate granular media withultra-small grain sizes via reactive oxidation/sputtering processing.

According to an illustrative, but non-limitative, embodiment of theinvention, each of the columnar-shaped magnetic grains comprises twooverlying sub-layers with different magnetic material composition. Afirst sub-layer at the first (upper) end of each of the magnetic grainsis comprised of CoCrX₁ first magnetic material, where X₁ is at least oneelement selected from the group consisting of Ta, Pt, B, V, C, Nd, Cu,Zr, Fe, P, O, Si, and Ni, with a magnetic moment M_(r) from about 200 toabout 800 emu/cc, a relatively high perpendicular coercivity H_(k) fromabout 8,000 to about 20,000 Oe, a thickness δ₁ from about 6 to about 25nm, and a grain size from about 4 to about 10 nm. A second sub-layer atthe second (lower) end of each of the magnetic grains is comprised ofCoX₂ second magnetic material, where X₂ is at least one element selectedfrom the group consisting of C, B, Cr, Pt, O, Fe, Ta, Cu, Nd, Ni, andTi, with a magnetic moment M_(r) from about 400 to about 900 emu/cc, arelatively low perpendicular coercivity H_(k) from about 1,000 to about9,000 Oe, a thickness δ₂ from about 3 to about 15 nm, and a crystalstructure and grain size matching those of the first sub-layer. Thetotal thickness δ₁+δ₂ of the first and second sub-layers is less thanthe exchange coupling distances of the magnetic materials, wherebydomain walls are not present in the magnetic grains. According to thisembodiment, a non-magnetic spacer layer is present at an interfacebetween the first and second sub-layers for providing an interfacialcoupling strength between the first and second sub-layers from about10⁻² to about 10⁻⁹ erg/cm, the spacer layer having a thickness up toabout 5 nm and comprised of at least one non-magnetic element selectedfrom the group consisting of Cr, Pt, Cu, Zr, V, C, Ru, Ta, and Si.

In accordance with another illustrative, non-limitative embodimentaccording to the present invention, each of the magnetic grainscomprises three overlying sub-layers with different magnetic materialcomposition, wherein the relatively high perpendicular magneticcoercivity H_(k1) of a first sublayer at the first (upper) end is about12,000 Oe, the relatively low perpendicular magnetic coercivity H_(k3)of a third sub-layer at the second (lower) end is about 3,000 Oe, andthe perpendicular magnetic coercivity H_(k2) of a second sub-layerintermediate the first and third sub-layers is about 9,000 Oe. Thethickness of each of the three sub-layers is about 6-8 nm.

According to yet another illustrative, non-limitative embodiment of thepresent invention, each of the magnetic grains comprises four overlyingsub-layers with different magnetic material compositions. The relativelyhigh perpendicular magnetic coercivity H_(k1) of a first sub-layer atthe first (upper) end of the columnar-shaped magnetic grains is about12,000 Oe, and the relatively low perpendicular magnetic coercivityH_(k4) of a fourth sub-layer at the second (lower) end is about 3,000Oe. The perpendicular magnetic coercivity H_(k2) of a second sub-layeradjacent the first sub-layer is about 9,000 Oe, and the perpendicularmagnetic coercivity H_(k3) of a third sub-layer adjacent the secondsub-layer is about 6,000 Oe. The thickness of each of the foursub-layers is about 5 nm.

It is noted that, while magnetic materials with coercivity values lessthan about 500 Oe are typically (or normally) characterized as softmagnetic materials and magnetic materials with coercivity values greaterthan about 2,000 Oe are typically characterized as hard magneticmaterials, all magnetic materials utilized in the present invention havea large intrinsic coercivity, i.e., >˜3,000 Oe, and thus would normallybe characterized as hard magnetic materials. Notwithstanding thischaracterization, the difference or variation between the intrinsiccoercivities and anisotropies of the component magnetic materials ofmedia according to the present invention can be fairly large, dependingupon the purpose or ultimate use of the media design and the recordinghead field gradient. The invention, therefore, is conceptually differentfrom merely combining hard and soft magnetic materials to form arecording medium. Rather, according to the underlying principle of thepresent invention, tailoring of the gradient of intrinsic perpendicularmagnetic coercivity/anisotropy, as well as the local exchange couplingstrengths of the perpendicular media are utilized in conjunction withthe recording head field strength to provide the media with maximum gainin SNR, thermal stability, and writability. Optimized media designsfacilitated by the present invention afford the smallest actualeffective head-media spacing (HMS), highest actual magnetic volumeK_(μ)V, and highest achievable writability at the effective volumeK_(μ)V.

In summary, the present invention provides perpendicular magneticrecording media fabricated with a main recording layer comprised ofcolumnar-shaped magnetic grains with specifically designed gradients ofmagnetic anisotropy, i.e., gradients of perpendicular magneticcoercivity (H_(k)), and with local exchange coupling strength(s) whichprovide good writability and signal-to-noise ratio (SNR) at ultra-highrecording densities (i.e., >˜250 Gbits/in²) and high data recordingrates (i.e., >˜2,000 Mbits/sec.) without significant sacrifice inthermal stability of the media. In addition, when the magnetizationreversal process is incoherent according to the invention, theread/write head spacing is reduced, as compared with the head-mediaspacing (HMS) with conventional coherent magnetization reversal,resulting in the improved SNR's, i.e., at least a 1-3 db increase in SNRfacilitating a corresponding increase in recording density.

In the previous description, numerous specific details are set forth,such as specific materials, structures, processes, etc., in order toprovide a better understanding of the present invention. However, thepresent invention can be practiced without resorting to the detailsspecifically set forth. In other instances, well-known processingmaterials and techniques have not been described in detail in order notto unnecessarily obscure the present invention.

Only the preferred embodiments of the present invention and but a fewexamples of its versatility are shown and described in the presentdisclosure. It is to be understood that the present invention is capableof use in various other combinations and environments and is susceptibleof changes and/or modifications within the scope of the inventiveconcept as expressed herein.

1-22. (canceled)
 23. A stack, comprising: a substrate having at leastone surface; and a layer stack on the surface of said substrate, thelayer stack including a perpendicular recording layer comprising aplurality of columnar-shaped magnetic grains extending perpendicularlybetween opposing surfaces of the recording layer, each columnar-shapedmagnetic grain comprising a plurality of sub-layers including: a firstsub-layer comprising a first magnetic material and having a firstmagnetic anisotropy field H_(k1); and a second sub-layer disposed on thefirst sub-layer, the second sub-layer comprising a second magneticmaterial and having a second magnetic anisotropy field H_(k2), whereinH_(k1)>H_(k2), and the second material differs from the first materialin one or both of composition and dopant concentration.
 24. The stack ofclaim 23, wherein the second material comprises CoPt.
 25. The stack ofclaim 23, wherein at least one of the first and second materialscomprises Fe.
 26. The stack of claim 23, wherein at least one of thefirst and second materials comprises Pt.
 27. The stack of claim 23,wherein each of the sub-layers has a perpendicular magnetic anisotropyfield that is greater than at least about 1000 Oe.
 28. The stack ofclaim 23, further comprising: a third sub-layer disposed on the secondsub-layer, the third sub-layer comprising a third magnetic material andhaving a third magnetic anisotropy field H_(k3), whereinH_(k1)>H_(k2)>H_(k3), and the third material differs from the first andsecond materials in one or both of composition and dopant concentration.29. The stack of claim 28, wherein H_(k1) is between 8,000 and 20,000 Oeand H_(k3) is between 1,000 and 9,000 Oe.
 30. The stack of claim 23,further comprising a non-magnetic, paramagnetic, or superparamagneticspacer layer disposed at an interface between at least one pair ofadjacent sub-layers for setting local exchange coupling between theadjacent sub-layers at a preselected strength.
 31. The stack of claim23, wherein the first sub-layer has a first thickness and the secondsub-layer has a second thickness and the sum of the first and secondthicknesses is less than local exchange coupling distances of the firstand second magnetic materials.
 32. A stack, comprising: a substratehaving at least one surface; and a layer stack on the surface of saidsubstrate, the layer stack including a perpendicular recording layercomprising a plurality of columnar-shaped magnetic grains extendingperpendicularly between opposing surfaces of the recording layer, eachcolumnar-shaped magnetic grain comprising a plurality of sub-layersincluding: a first sub-layer comprising a first magnetic material andhaving a first magnetic anisotropy field H_(k1); and a second sub-layerdisposed on the first sub-layer, the second sub-layer comprising asecond magnetic material and having a second magnetic anisotropy fieldH_(k2), wherein H_(k1)>H_(k2).
 33. The stack of claim 32, wherein thefirst material and the second material have different compositions. 34.The stack of claim 33, wherein the second material comprises CoPt. 35.The stack of claim 32, wherein the second magnetic material differs fromthe first magnetic material in dopant concentration.
 36. The stack ofclaim 32, wherein at least one of the first and second materialscomprises Fe.
 37. The stack of claim 32, wherein at least one of thefirst and second materials comprises Pt.
 38. The stack of claim 32,wherein each of the sub-layers has a perpendicular magnetic anisotropyfield that is greater than at least about 1000 Oe.
 39. The stack ofclaim 32, wherein each of the sub-layers comprises a cobalt-based alloy.40. An apparatus, comprising: a columnar-shaped magnetic graincomprising: a first magnetic layer comprising a first magnetic materialand having a first magnetic anisotropy field, H_(k1); a first spacerlayer adjacent to the first magnetic layer; a second magnetic layeradjacent to the first spacer layer, the second magnetic layer comprisinga second magnetic material and having a second magnetic anisotropyfield, H_(k2); a second spacer layer adjacent to the second magneticlayer; and a third magnetic layer adjacent to the second spacer layer,the third magnetic layer comprising a third magnetic material and havinga third magnetic anisotropy field H_(k3), wherein each of the first,second, and third magnetic materials differs from the other magneticmaterials in one or both of composition and dopant concentration, andH_(k1)>H_(k2)>H_(k3).
 41. The apparatus of claim 40, wherein at leastone of the first, second, and third magnetic materials comprises Pt. 42.The apparatus of claim 40, wherein at least one of the first magneticlayer, the second magnetic layer, and the third magnetic layer comprisesat least one of a Co alloy, alternating layers of a Co alloy and a Ptalloy, or alternating layers of a Co alloy and a Pd alloy.
 43. Theapparatus of claim 40, wherein each of the sub-layers has aperpendicular magnetic anisotropy field that is greater than at leastabout 1000 Oe.
 44. The apparatus of claim 40, wherein H_(k1) is between8,000 and 20,000 Oe and H_(k3) is between 1,000 and 9,000 Oe.